Semiconductor and optoelectronic packaging encompasses various processes involved in assembling, connecting, and bonding components of semiconductor or optoelectronic devices. As the sizes of such devices have become increasingly miniaturized, adhesives are increasingly being used for packaging applications in place of mechanical connecting elements, such as clamping devices and threaded connections. Small semiconductor and optoelectronic devices typically require precise alignment of components with fine tolerances (usually <1 micron), so-called “active alignment” by those in the industry, such that adhesives used for packaging small semiconductor and optoelectronic devices should be capable of meeting such processing demands.
Moreover, as such devices frequently include shaded regions between components which see limited to no light exposure, current adhesives used in semiconductor or optoelectronic packaging applications may be “dual cure” adhesives which are curable both photochemically and thermally. In particular, portions of the adhesive that are exposed to light may be cured photochemically (e.g., with UV light), sometimes within seconds, to fix the components together in alignment within a desired tolerance limit. The remaining portions of the adhesive that are buried from light in the shaded regions are then heat treated to complete the curing process. However, in the case of camera modules, for example, miniaturization has also led to the use of high refractive index lenses fabricated from polycarbonate (PC), modified PC, or other similar plastics that may melt or deform when exposed to high temperatures (>90° C.). Accordingly, current dual cure adhesives used for such applications should be capable of curing at relatively low temperatures to maintain the integrity of heat sensitive materials.
Current dual cure adhesives used for packaging of electronic/optoelectronic devices, including camera modules, are often hybrid resins containing an acrylate, acrylated bis-maleimide (BMI), acrylated urethane, or other similar acrylated resins that cure by radical polymerization in the presence of low temperature heat, and an epoxy resin that cures by acid-catalyzed polymerization when exposed to UV light. However, although epoxy adhesives are well known to outperform acrylate and BMI adhesives in terms of superior adhesion and environmental properties (e.g., water permeation, etc.), there are currently no known commercially available dual cure, epoxy only adhesives that are curable both photochemically and thermally at low temperatures needed for temperature sensitive components, such as polycarbonate lenses.
Epoxy adhesives may be grouped into two categories depending on how curing is initiated: thermally curable epoxy adhesives, and photochemically curable epoxy adhesives. Thermally curable epoxy adhesives may be further divided into two groups depending on the polymerization mechanism. Specifically, thermally curable epoxy adhesives may be grouped into those that polymerize by condensation of an epoxide with another functional groups such as an amine or an anhydride (so-called “two-part” systems), and those that undergo acid-catalyzed polymerization (single component systems). The two-part epoxide condensation systems are incompatible with acid-catalyzed polymerization due to their basic components.
U.S. Pat. Nos. 4,225,691 and 4,238,587 describe resin compositions containing a polymerizable epoxy resin, diaryliodonium salts, and copper-based redox agents that undergo thermal curing at low temperatures via acid-catalyzed polymerization. While effective, the resin compositions disclosed therein are so reactive that they may gelate in a matter of minutes or hours. Thus, such epoxy resin compositions may be difficult to handle, and should be prepared immediately before use.
Thus, it can be seen that there is a need for dual cure, all epoxy adhesives that can be stored at room temperature for significant periods without gelating. Moreover, there is a need for such adhesives that are compatible with the processing demands of active alignment, and are curable at low temperatures for temperature-sensitive applications such as small camera module assembly applications.